1. Field of the Invention
The present invention relates to implantable intervertebral implants and fasteners particularly useful for assisting with the surgical arthrodesis (fusion) of two spinal vertebrae and more particularly, to an anchoring system that provides and controls limited movement between vertebrae during fusion.
2. Background Art
Various genetic or developmental disorders can affect the structure and function of the spinal column. Trauma or advancing age can lead to changes in the bones, disks, joints, and ligaments of the spine producing pain. Under certain circumstances, alleviation of pain can be provided by performing a spinal arthrodesis, commonly known as a spinal fusion. This procedure is accepted and performed by the spinal community and involves joining two or more adjacent vertebrae so that they are no longer able to move relative to each other.
Many prosthetic devices are known for promoting fusion of the spinal vertebrae. The spine surgical community has accepted intervertebral devices—commonly known as interbody spacers—as part of the state of the art and routine practice employs such devices for spinal arthrodesis. Surgeons insert these intervertebral devices to facilitate bone fusion in between and into the contiguous involved vertebrae. This fusion creates a new solid bone mass, which acts to hold the spinal segment at an appropriate biomechanically restored height as well as to stop motion in a segment of the spine in which the patient is experiencing pain. Items surgically placed in these involved interbody regions can thus stimulate interbody bone in-growth such that the operated anterior spinal segments heal into a contiguous bone mass; in other words, a fusion occurs.
These prosthetic devices may be classified, in part, based upon the approach to the spine through which they will be inserted (anterior, lateral, posterior, etc). They may also be classified based upon their mechanism of spinal fixation (separate plate and screw fixation, incorporated blade/screw fixation, incorporated screw fixation without plate, blade fixation without plate). When referenced throughout this disclosure, the term “blade” can be considered to include a blade, a nail, an anchor, and/or a non-threaded screw.
There are several commercially available devices that operate as stand-alone (that is, without support from an additional construct such as an anterior plate and screws, or posteriorly placed screws and/or rods placed into the pedicles or facet joints) interbody fusion devices. These devices include the Stalif™, SynFix™, Zero-P™, and the VerteBridge™. The Stalif™ is a device for the fusion of the lumbar spine. The implant is inserted and fixed via diverging screws passing through pre-drilled apertures of the device that penetrate into the vertebral bodies without the use of a plate or locking of the screws. The screws are manually placed into the apertures of the device and are driven using an appropriate tool, such as a surgical screw driver. The SynFix™ is also a device for fusion of the lumbar spine that is placed in an intervertebral space and fixed via diverging screws passing through the device and into the vertebral bodies. Again, the screws are manually placed into the apertures of the device and are driven using a surgical screw driver. The Zero-P™ is a cervical fusion device which also fixed via diverging screws passing through the device and into the vertebral bodies. Again, the screws are manually placed into the apertures of the device and are driven using a surgical screw driver. The VerteBridge™ is a device for the fusion of the cervical or lumbar spine in which anchoring blades are press-driven through apertures in the device and into the respective vertebral bodies to fix the device in place without the use of a plate.
All of the above-described devices have an anchor which is secondarily added to the initial device. The Stalif™, SynFix™, and Zero-P™ devices employ screws while the VerteBridge™ utilizes a blade anchor. Both the Stalif™ and SynFix™ devices require the screws to be inserted at trajectories that are difficult to achieve given common human anatomical structures, especially at the spinal level L5-S1. Additionally, the proximal end of the screws may protrude anteriorly, causing potential irritation to the great vessels overlying the lumbar spine. Due to the problematic angulation of the screw trajectory, hinged screw drivers are provided for their insertion. These difficult trajectories increase the difficulty of insertion and increase the risk of injury to surrounding structures. Rigid fixation through the SynFix™ or the Zero-P™ does not allow for the interbody graft to heal in compression which increases the chance of pseudoarthrosis (failure of fusion). The VerteBridge™ has a pair of blades inserted after the initial device is put in place. These blades have a locking mechanism which cannot be released if removal of the implant is required. The locking mechanism is supposed to be of sufficient strength to prevent failure and backout of the anchor to prevent irritation or injury to the great vessels overlying the lumbar spine or the esophagus overlying the cervical spine. Additionally, the blade anchors are supposed to exhibit sufficient biomechanics to allow appropriate segmental immobilization to promote solid arthrodesis. In practice, these features are not always achieved.
There are several commercially available devices that operate as anchoring devices placed in a direct lateral position in the lumbar spine. The XLP™, the ORACLE™, and the ZUMA™ employ a plate and screws to stabilize the anterior spine when an intervertebral device is placed through a direct lateral transpsoas approach to the lumbar spine. These fixation devices are placed over the lateral vertebral body and have the disadvantage of risk to the traversing nerve roots which pass through this region. The ORACLE™ and ZUMA™ plating systems employ a four-hole plate for fixation and the width of the plate makes safe placement through this lateral region of the spine difficult. The XLP™ is a two-hole plate that minimizes this risk but the plating system also encroaches on these traversing nerve roots. There currently is no stand-alone interbody fusion device that provides direct lateral lumbar fusion with biomechanical fixation while still protecting the nerve roots through this approach.
Further conventional devices are described by Donner/Synthes (4-2009) wherein an Arcuate Fixation Member is described. Despite the accurate description of a curvalinear anchor with multiple fixation purposes, the described device is lacking in several areas as it relates to interbody fusion. First and foremost, the broad description of the arcuate anchor lacks critical specifications. Relative to interbody fusions, regardless of whether the anchor is curvalinear in a continuous or noncontiguous fashion, the final angulation relative to the horizontal plane may be critical. If the resulting angulation is not within 50-90 of the horizontal plane, the device cannot be expected to improve that currently available. Further, the mating boring guide will loosen the anchor and its fixation from the outset, and the tapered shaft will decrease bone surface area contact and fixation. The device also lacks guiding splines on the blades with a matching portal through the fixation plate for reproducible accurate blade placement. Absent also is a top loading introducer for matching the portal through the plate to overcome the current difficulty with screw trajectory. This is perhaps the most important aspect as patient safety is at risk with the use of hinged screwdrivers, drills, and awls. Thus, the arcuate fixation member will fail to overcome the problems inherent with the current devices available.
It may be desirable to provide an intervertebral device for the fusion of the cervical or lumbar spine in which anchoring blades are placed through apertures in the device and into the respective vertebral bodies to fix the device in place with the use of a plate. This device would utilize helical or non-helical angled blades which are locked to a plating system to prevent back-out and risk to the great vessels overlying the lumbar spine, and prevent risk to the esophagus overlying the cervical spine. This plate would also avoid risk to the traversing nerve roots through the direct lateral or transpsoas approach to the lumbar spine. This anchoring system may have the benefit of functioning as a stand-alone intervertebral device or as a separate plating device by itself. This intervertebral device would allow for safe removal if necessary. This device would also overcome the problems encountered with the current art requiring screws to be inserted at trajectories that are difficult to achieve given common human anatomical structures and would not require insertion with screw drivers or hinged screw drivers. This device would also allow fusion segments to heal in compression, allowing for increased fusion rates as a result of the blade design.
It may be desire to provide an intervertebral device that would utilize angled blades with either linear or helical splines. The curvalinear radius of the splined blade anchor may result in a terminal angulation of 50-90 degrees as measured from the horizontal plane. This feature is disregarded in the prior art ignoring Wolfs law which states that bone heals best in compression. It may be further desirable to provide a device without a slidably mating curved guiding bore through which the curved anchor is inserted, as this may cause the curved bone anchor to be inserted loosely. There will instead be no need for this as the configuration of the splined blade and its mated portal through the fixation plate will guide the anchor to its ideal position without compromising tight fixation.
It may be desirable to provide a device with a minimum of two fixation blades, and in some aspect preferably three. This may afford more adequate biomechanical fixation and allow use at multiple consecutive vertebral levels as well as levels above or below previously instrumented vertebral segments. Superior and inferior Blades will be oriented in a directly opposing manner as referenced from the horizontal plane optimizing compressive subsidence of vertebral segments.
It may be desirable to provide a device with a minimum of two splines orientated in either a linear or helical fashion with respect to the longitudinal axis of the blade whose purpose is to guide the blade accurately to its correct position within the bone. There may be no taper between the proximal and distal ends allowing for larger contiguous surface area for bone fixation. The head or proximal end of the blade may have the same spline orientation as the shaft so there will be no change in course throughout the entire length of the blades insertion through bone. The proximal end and head may not only have same spline orientation but may also have a morse taper matching the guiding chamber of the plate. The proximal end, head, with the features above, may seat into the plate such that the spines would be below the anterior surface of the plate allowing for a single cover screw to prevent backout if the morse taper was felt to be inadequate.